Moments before he was put to death, Troy Davis lifted his head from the gurney to which he was strapped and looked the family of Mark MacPhail, the police officer for whose murder he was convicted, directly in the eyes.

"I want to talk to the MacPhail family," he said. "I was not responsible for what happened that night. I did not have a gun. I was not the one who took the life of your father, son, brother."

He then appealed to his own family and friends to "keep the faith", said to the medical personnel who were about to kill him "may God have mercy on your souls", and laid his head down again.

He was administered with a triple lethal injection of pentobarbital, pancuronium bromide and potassium chloride, and at 11.08pm he was pronounced dead.

The debate about what happened in Georgia's Diagnostic and Classification Prison in Jackson late on Wednesday night will continue long after the gurney has been put away. In the final gruesome hours of waiting, the American judicial system at its very highest echelons was involved – including the US supreme court, which issued the decisive final ruling. The decision to press ahead with the death sentence despite serious doubts over Davis's guilt drew accusations that this was the system at its most grotesque.

It was Davis's fourth execution date, and it was dragged out, for more than four hours, to what must have been tortuous effect for the prisoner and his family.

Davis, 42, became the 52nd man to be executed in Georgia since the same supreme court reinstated the death penalty in 1973. His lawyers and thousands of supporters around the world were convinced that an innocent man had been sent to his death.

As news of his death filtered out of the maximum security prison, his family was still huddled in an area of the prison grounds, surrounded by well-wishers. His sister, Martina Correia, had earlier vowed to continue the fight to end all capital punishment in America and said her brother's story would be a galvanising force for others.

"His message to young people is – you can lie down or you can stand up and fight," she said.

After the execution, Davis's lawyers lamented what one described as a "legal lynching". Thomas Ruffin said that the execution was "racially bigoted".

"In the state of Georgia 48.4% of people on death row this morning were black males, and in Georgia they make up no more than 15% of the population."

Ruffin said that seven of the nine witnesses at Davis's 1991 trial had since recanted. They included a man who said under oath that he had seen MacPhail being killed, and that it was not Davis who shot him but another man called Sylvester Cole.

Another witness said under oath that she had heard Coles confess three times to killing MacPhail and using Davis as the fall guy.

But throughout Wednesday last-ditch legal manoeuvres by Davis's defence team failed one by one as both state and federal judges ruled against them. First, a request for Davis to undergo a lie-detector test was rejected. A Georgia judge refused an appeal, then the state supreme court followed suit. In the end his only hope was the US supreme court, and for a moment at 7pm, just as the execution was due to take place, it seemed that the justices had ordered a stay. But the delay was only temporary and at around 10.15pm the court ruled that the execution should go ahead.

The decision was welcomed by the MacPhail family. "He [Davis] had all the chances in the world," the officer's mother, Anneliese MacPhail, told the Associated Press. "It has got to come to an end."

But a second lawyer for Davis, Jason Ewart, said that as he died he took with him "his quest for justice. In the midst of all the newspaper headlines and vigils you can sometimes lose sight of the man who was on death row. Troy Davis was a family man, and his family mourns tonight."

John Lewis, a local radio reporter who was present at the execution, said that while the prisoner was being killed MacPhail family members sat in the front row looking intently at him. As they left the room after he was pronounced dead, some of them smiled.